Tunnel design in challenging hydrogeological conditions: water inflow analysis during rock excavation and mitigation measures Application to the Rv.555 Sotra Connection project in Norway

During the excavation of the tunnel one of the greatest problems that can be encountered is related to the presence of groundwater. One issue that can arise from that, is the possibility of having uncontrolled volumes of water incoming into the underground excavation. Dealing with rock masses, critical inflows can be encountered when the excavation crosses discontinuities, weathered zones or faults. The degree of the inflow will depend on the aperture and other properties of these discontinuity zones being the intact rock an impervious medium. Water inflow in addition to creating unpleasant conditions underground for the workers, it also can cause problems of settlements on the surface. In order to avoid such phenomena from happening, in sections of tunnels where the water inflow is estimated to be higher than the maximum water inflow that can be accepted, some mitigation measures such as pre-grouting can be adopted. Therefore, the definition of maximum water inflow limits is mandatory to create a threshold. These limits will be defined for each section according to the severity of the effects that groundwater inflow could have on the surface in an area surrounding the tunnel. The threshold will subsequently be compared to the expected inflow in the section to determine if grouting is necessary or not. Finally, in critical sections, where grouting is needed, numerical modelling can be carried out to better understand the effectiveness of grouting both in terms of water income and settlements.